This invention relates to servo track following of servo edges of dissimilar servo signals, and, more particularly, to calibrating indexed servo positions displaced with respect to the servo edges, where the sensed dissimilar servo signals may differ in amplitude from nominal.
Magnetic tape data storage typically provides prerecorded servo tracks to allow precise positioning of a tape head which has servo sensors, with respect to the prerecorded servo tracks. The tape head comprises one or more read/write elements precisely positioned with respect to the servo sensors and which trace data tracks parallel to the servo tracks. One example of a magnetic tape system is the IBM 3590, which employs magnetic tape having prerecorded servo patterns that include three parallel sets of servo edges, each servo edge being an interface between two dissimilar recorded servo signals, each set of servo edges comprising one servo edge on each of opposite lateral sides of a middle recorded servo signal.
The tape head has several spaced apart servo sensors for each servo edge, with the result that the tape head may be stepped between the servo sensors, each positioning the read write elements at different interleaved groups of data tracks.
Typically, for a given servo pattern of a set of two servo edges, the outer servo signals are recorded first, and the center servo signal is recorded last, to provide the servo edges. As pointed out by the incorporated ""159 patent, the nominal separation distance between the servo edges of each set of servo edges is a certain distance, such as 80 microns, but there is variation in the magnetic separation between the servo edges, for example, due to the variation of the width of the physical write element which prerecords the servo pattern, due to variation in the magnetic characteristics of the physical write element, etc. The variation may occur between servo tracks in a single magnetic tape, and may occur between prerecording devices and therefore between magnetic tapes.
To reduce the apparent difference of the edge separation distance of the prerecorded servo tracks from nominal, the prerecording of the servo tracks is conducted at different amplitudes so as to attempt to compensate for the physical difference and provide a magnetic pattern that is closer to nominal. Additionally, three servo sensors are employed to simultaneously sense the three servo tracks, and, the average of the servo signals may be employed to track follow the servo tracks. Thus, the difference in physical distance and in amplitude compensation may tend to offset as between the servo tracks. These actions may provide an adequate signal for track following at the servo edges.
However, to increase track density, the servo sensors may themselves be indexed to positions laterally offset from the linear servo edges to provide further interleaved groups of data tracks. The indexed positions are determined by measuring the ratio between the amplitudes of the two dissimilar recorded servo signals. Thus, when the amplitudes of the recorded servo signals are varied to compensate for physical distance variations, track following the prerecorded servo edges at the offset indexed positions becomes less precise. As the result, the data tracks may vary from the desired positions, for example, squeezed together, such that writing on one track with a write element that is subject to track misregistration (TMR) may cause a data error on the immediately adjacent data track.
It is an object of the present invention to calibrate the servo index positions which are laterally offset from the servo edges comprising interfaces between two dissimilar recorded servo signals.
Disclosed are a servo system and method for calibrating servo index positions of a magnetic tape for track following, where the magnetic tape has a plurality of parallel sets of linear servo edges, each servo edge comprising an interface between two dissimilar recorded servo signals, each set of servo edges comprising one of the servo edges on each of opposite lateral sides of a middle recorded servo signal. At least some of the servo index positions are laterally offset from the servo edges.
The servo system has at least one servo sensor of a tape head, where the tape head is movable laterally of the magnetic tapes the servo sensor sensing the servo signals comprising the servo edges of at least one set of linear servo edges. An independent position sensor determines the mechanical lateral position of the tape head servo sensor with respect to a tape deck supporting the magnetic tape, and therefore the magnetic tape. A servo detector is coupled to the servo sensor for determining a ratio of the servo signals sensed by the servo sensor, and a servo loop is coupled to the servo detector for positioning the tape head laterally of the magnetic tape to track follow the sensed servo signals at specific position error signals representing offset displacements from the linear servo edges in accordance with the ratios of the sensed servo signals as determined by the servo detector.
Logic is coupled to the servo detector, the independent position sensor, and the servo loop. In accordance with the present invention, the logic operates the servo loop to laterally position the servo sensor to sense the servo signals at estimated ratios of the servo detector representing locations comprising the servo edges of at least a set of linear servo edges. Employing the independent position sensor, the logic determines the lateral position of the tape head servo sensor with respect to the magnetic tape at the sensed servo edges. The logic operates the servo loop to laterally reposition the tape head servo sensor to a plurality of lateral positions of the tape head at predetermined displacements from the sensed servo edges as determined by the independent position sensor, and to track follow the linear servo edges with the servo loop at each of the predetermined displacements. The logic determines, from the servo detector, the ratio of the sensed servo signals at each of the predetermined displacements, and curve fits a two dimensional, second order curve to the ratios of the sensed servo signals with respect to the predetermined displacements for each linear servo edge to calibrate expected position error signals for the servo loop at the laterally offset servo index positions with respect to the sensed servo edges of the set of linear servo edges.